The objective of this proposal is to ascertain the site(s) of action of progesterone in the induction ovulation of the laying hen (Gallus domesticus) and to determine some aspects of the interaction between photoperiod and timing of ovulation, on the one hand, and the sensitivity of the hypothalamus to progesterone stimulation and of the anterior pituitary to LH-RH stimulation, on the other hand. Five experiments are planned. The first is designed to test the hypothesis that the sensitivity of the hypothalamus to intraventricular progesterone injection shows a daily variation. In the second and third experiments the hypothesis is tested that progesterone may act on the largest follicle. In the fourth experiment the effect of estrogen, progesterone, and testosterone on the responsiveness of the hypophysis to exogenous LH-RH in ovariectomized hens is tested. In the fifth experiment the effect of time on the response of the hypophysis to 20 ug and 40 ug LH-RH is ovariectomized hens given optimal doses of E, T and P (based on the results of Experiment 4) will be determined. In these experiments measurements of LH, E1, E2, P, C and T will be made and the time of ovulation will be determined by digital palpation.